Conventionally, as information recording media used for computers, magnetic disks are available. As magnetic disk substrates, aluminum substrates have been commonly used. However, over recent years, with the demand for reduction of magnetic head floating amount for enhancing recording density, glass substrates are used at an increasing percentage, with which glass substrate the magnetic floating amount is reduced because glass substrates exhibit superior surface flatness and have fewer surface defects than aluminum substrates.
These glass substrates of magnetic disks are produced by polishing glass substrates referred to as blank materials. It is known that such glass substrates (blank materials) are produced by a production method employing press molding and a production method in which plate glass produced by a float method is cut. Glass substrates cut out in a certain shape have large surface non-uniformity as such. Therefore, the surface needs to be polished. Further, to meet the demand for higher density, the technology of high precision polishing is required.
As well known conventionally, the process of surface finishing of glass roughly contains the following steps: a first lapping step; a second lapping step; a first polishing step; and a second polishing step (for example, refer to Patent Document 1 and Patent Document 2).
The first lapping step is a step in which both surfaces of a glass substrate is polished to preliminarily adjust the parallelism, the flatness, and the thickness of the glass substrate. The second lapping step is a step to fine-tune the parallelism, the flatness, and the thickness of a glass substrate.
The first polishing step and the second polishing step are so-called polishing steps. In the first polishing step, scratches and defects are eliminated and the surface of a glass substrate is allowed to have predetermined surface roughness. Thereafter, in the second polishing step, the surface of the glass substrate is polished further precisely. Recently, since a glass substrate is required to have high-level flatness, in both the first polishing step and the second polishing step, a polishing method employing a polishing liquid containing polishing agent is used.